1. Field of the Invention
This invention relates to a porous polytetrafluoroethylene structure that can be formed into an implanted prosthesis with improved physical strength and surgical handling (kink and compression resistance, and ease of tunneling during surgical placement), along with improved mechanical performance (resistance to dilation and physical strength degradation) in arteriovenous applications. It also relates to a method of manufacture of that structure.
2. Description of the Prior Art
Conventional vascular grafts manufactured from porous polytetrafluoroethylene have limitations in surgical handling and healing. In some instances the porous grafts are wrapped with an external reinforcing film to increase radial strength. Vascular grafts may also be reinforced with an external spiral bead or ring. The reinforcing film does not provide radial support to prevent kinking and collapse during placement or during access use. Furthermore, the presence of an external bead or ring results in interference during surgical placement increasing trauma to the surrounding tissue. In addition, such grafts may be stiff and noncompliant to the natural artery.
Surgical implantation procedures require placement of the vascular graft within the subcutaneous tissue of humans. Peripheral and angioaccess vascular procedures require an anatomic or subcutaneous pathway commonly called tunneling. Tunneling is an initial surgical step in the vascular procedure which can result in localized injury to adjacent tissue. The tunnel diameter relative to the implant diameter, as well as the abrasive force exerted by the implant to the adjacent tissue have a significant impact on the resultant healing response.
It is advantageous in the clinical setting to minimize trauma through ease of tunneling. One approach is to use an expensive surgical tool that often results in larger than required pathways influencing the healing response by creating a fibrous capsule that surrounds a fluid sac that does not incorporate the implant.
One problem which can arise with current PTFE arteriovenous grafts is a lifespan limitation due to physical attrition of the graft caused by poor dialysis access technique identified by repeated needle punctures in concentrated areas resulting in ever enlarging holes or tears in the material comprising the graft wall. Maturation of the surrounding tissue incorporating a vascular access graft, to reduce the adventitial space between tunnel and implant, is a prerequisite to use of the graft for subsequent use in dialysis. The maturation time is necessary to prevent tunnel hematomas which can occur from premature graft puncture. For this reason, it is currently recommended that 1 to 4 weeks pass before initial needle puncture is performed.
Broadly speaking, the present invention provides for an implantable multistage structure which has integral reinforcement within the device wall having properties that allow for improved surgical handling at implantation, reduced tissue trauma to provide improved healing, and improved performance in an arteriovenous device, together with a method for making the same.
The implantable multistage PTFE porous structure of the invention includes an integral circumferential support within the cross-section with one or more thickness zones within the cross-section having smaller than average pore diameter than the other sections, and in which all the zones have been bonded to the adjacent zones completely throughout the interfaces, free of interlaminar peeling.
The multi-stage structure may be in the shape of any suitable medical implantable device. However, the structure of the invention is particularly advantageous when in the form of an implantable tubular prosthesis, such as a vascular graft.
One embodiment of the present invention includes in vivo implantable structures formed with two or more zones of different node/fibril geometry with an integral intrazone circumferential support. An object of this invention is to provide shaped products manufactured from PTFE that are biologically compatible with surrounding tissue. Another object of the present invention is to provide an in vivo implantable material having improved surgical handling and implant performance.
The biologically compatible material of the present invention has excellent compatibility, strength, and surgical handling because of the arrangement of integral support and node/fibril PTFE fibrous structures. Some current vascular prostheses are designed with an external biaxially oriented reinforcement wrap, spiral bead, or ring, in direct contact with adjacent tissue, to provide additional radial strength to a tubular product, but which results in poor surgical handling during placement and poor compliance. Tubes of the present invention provide improved surgical handling during placement which results in quick maturation and tissue incorporation leading to good healing. In addition, tubes of the present invention provide for greater needle holes per unit area without physical strength compromise in order to address the problem of premature physical failure due to poor cannulation technique.
The products of the present invention have a very broad application in medical devices, such as vascular grafts, endovascular devices, and vascular access devices. In a preferred embodiment, each radial cross-section region of the implant can be distinguished from other regions by having different pore size, pore shape, and porosity in conjunction with an intrawall circumferential support integral to the structure. Indeed, the fibril-nodal microstructure throughout the matrix may have the internodal distance, i.e. pore size, in one section at least two to twenty times that for its adjacent sections. One in vivo material has two cross-section regions. The first region, for example, has an internodal distance of the pores of the PTFE luminal surface of about 20 or 30 microns and a specific node/fibril geometry. In the next zone the internodal distance of the pores is a range from about 1 to about 10 microns and a specific node/fibril geometry, preferably 1 to 5 microns. This pore size is excellent for cell growth mediator permeability, instead of undesired encapsulation. Another embodiment of the present invention includes the luminal surface and second and third zones of material previously described whereby the third zone has a pore size range of 50 to 500 microns and a specific node/fibril geometry, preferably about 50 to 100 microns which is excellent for fibroblast tissue ingrowth, as the healing process progresses. In a further embodiment, a circumferential support having a radius of diameter from 25 to 100 microns is present within the wall structure to provide kink and compression resistance along with dialysis technique improvement.
As discussed above, one embodiment of the present invention includes an in-vivo implantable material comprising the luminal, second, and third regions in combination with an integral circumferential support previously described. Another embodiment of the present invention includes the luminal, second and third region of material previously described with the third region or the integral support providing a source location for drug delivery.
In a still further preferred embodiment of this invention, a fluoropolymer bead is wrapped around the outer surface of the composite structure under tension. This embodiment is particularly useful in the preparation of vascular grafts. That is, the multistage structure is a tubular shaped structure with maximum compression resistance having particular utility in applications where such properties are extremely advantageous, (i.e., peripheral bypass surgery, endoluminal).
The above described devices do not have to be totally implanted within the body to be considered within the scope of the present invention and include, among other devices, catheters, transcutaneous tubing or artificial skin.